An emergent theme in the study of aggression is that multiple hormonal and neuromodulatory systems influence behavioral output. However, mechanisms whereby these systems interact remain largely unknown. In Homarus americanus, the American lobster, a robust behavioral paradigm has been established for the quantification of aggression, and several neuromodulatory systems have been mapped. The fact that individual neurosecretory cells can be identified uniquely from animal to animal in this species will be exploited to uncover basic principles about regulation of co-transmitter release and interactions at the single-cell level between neurohormonal systems that modulate complex behaviors. The monoamines serotonin and octopamine and the pentapeptide proctolin have been implicated in the modulation of aggressive behavior in decapod crustaceans. These neuromodulatory systems converge at the second thoracic roots, nerve bundles which carry the prodigious neurosecretory endings of octopamine- and serotonin/proctolin-containing cells to the general circulation. Somata of neurons containing Crustacean Hyperglycemic Hormone (CHH), a putative stress hormone, are located at branch points in the second thoracic roots. This research proposal consists of experiments designed to examine the regulation of the CHH neurons by the other identified neurosecretory cells.